It is well known to make particulate bodies for a variety of applications from ceramic or metal equiaxed and/or fibrilose particles by: (1) molding a mixture of the particles and an all organic temporary binder (e.g., wax) to provide a green mass; (2) heating the green mass to remove the temporary binder; and finally, (3) heating the temporary-binder-free mass sufficiently (i.e., time and temperature) to sinter or otherwise bond the particles together into a self-supporting article. Such a technique has been used to make preforms for making "metal matrix composites" (i.e., MMCs) which comprise filler particles dispersed throughout a matrix metal such as Al, Mg, etc. Where the filler particles serve to improve one or more of the mechanical properties (e.g., strength, toughness, lubricity, friction, wear resistance, etc.) of the MMC over the properties of the matrix metal alone. Popular fibrils for making MMCs have an average aspect ratio (i.e., length divided by diameter) of at least 10 to 20, but may be as low as 3 and as high as about 200. The lengths of the fibrils vary from about 10 microns (though smaller are possible) to about 500 microns, and their diameters are generally less than about 10 microns. The filler fibrils will typically constitute about 5% by volume to about 45% by volume of the MMC with the balance being the matrix metal. More specifically, it is known to make MMCs by a process wherein a self-supporting, net shape, porous preform is made from the filler particles, and the preform subsequently infiltrated with the matrix metal by well known wicking or pressure filling (e.g., squeeze casting) techniques. The preform itself is made by molding a mixture of the filler particles and an all organic binder to a desired size and shape, removing the organic binder, and then bonding the particles together. One particular such molding process for making MMC preforms comprises: (1) mixing the filler particles uniformly throughout a fugitive organic binder/vehicle (e.g., wax, polystyrene, polyethylene, etc.) which comprises about 55% to about 95% by volume of the mixture; (2) injecting the binder-particle mixture under pressure into a mold; (3) burning-out/volatizing the fugitive binder; and finally, (4) bonding the particles together into a self-supporting structure. One such process is disclosed in Corbett et al U.S. Pat. No. 5,335,712. Final bonding of the particles may be achieved: (1) by sintering; (2) by initially providing the particles with a coating of colloidal silica or alumina which, upon heating, acts like a high temperature inter-particle glue; or (3) in some cases, by oxidizing the particles to hold them together. In this latter regard, SiC particles can be bonded together by heating the particles to above 600.degree. C. in air to form SiO.sub.2 in situ on the surfaces thereof which serves to bond the particles each to others.
One problem with the aforesaid techniques is that superambient injection temperatures were needed to melt and volitize the large volume of binder. Another problem is that the articles so made tend to distort (e.g., slump) when heated to remove the binder which typically softens before volatizing. This often happens because the binders are thermolastic and melt before volatizing and/or pyrolyzing. Finally, the time, cost, and environmental considerations associated with burning off large volumes of the organic binder reduce the commercial attractiveness of the process.
Water-based vehicles for the particles have been proposed. In one variant thereof, the vehicle comprises a methycellulose-H.sub.2 O gel, wherein a mixture of the particles, methyl cellulose and water are injected into the mold cavity and the mixture heated to about 80.degree. C. in the mold cavity to cause gelling of the methyl cellulose. The gel-bound particles are then removed from the mold and heated to remove the water and the methyl cellulose. In the gelled state, the methyl cellulose is quite weak and often cannot withstand ejection from the molding die or subsequent drying outside of the die. Maxwell et al, U.S. Pat. No. 2,893,102, discloses a freeze-cast process wherein porous bodies are made from equiaxed particles by: (1) forming a thick, moist, nonflowing slip of the particles; (2) injecting the slip with vibration- and vacuum-assisted hand pressure into a mold cavity; (3) freezing the molding in the cavity; (4) removing the frozen molding from the cavity; (5) freeze-drying the molding; and (6) sintering the particles together. The slip contains only a small amount of slip agent (e.g., 3.5 mils of H.sub.2 O per 20 gs of particles), and may contain a small amount (e.g., 2% of the weight of the particles) of a binder (e.g., starch or gelatins) to provide some green strength after the water is removed. In another variant of the freeze-cast process according to Occhionero et al., U.S. Pat. No. 5,047,181, porous bodies are made from primarily equiaxed particles by (1) forming a low viscosity, pourable, aqueous slip of the particles; (2) injecting the slip under low (i.e., &lt;200 psi) pressure into a mold cavity, (3) freezing the molding in the cavity, (4) removing the molding from the cavity, (5) freeze-drying the molding, and (6) sintering the particles together. The pourable slip (i.e., viscosity &lt;10 poise at 100 sec.sup.-1 shear rate) contains at least 35% by volume ceramic or metal particles, and an aqueous vehicle containing a dispersant and a cryoprotectant (which may also be a dispersant, a green strength enhancer and/or viscosity modifier such as methylcellulose or ethylcellulose).
The aforesaid freeze casting processes are incapable of forming fibrillose bodies having a substantially uniform distribution of fibrils throughout. In this regard, when using Maxwell's essentially "dry" (i.e., low water content) process, the intertwining of the fibers prevents their being injected into, or distributed uniformly throughout, the mold cavity under the hand pressures described therein. On the other hand, when using Occhionero's low viscosity process, a uniform moldable mix cannot be produced as the slip agent (i.e., H.sub.2 O) is so fluid that, even with extensive mixing, it will not break up the fiber bundle raw materials into discrete fibers and distribute them evenly throughout the mix so as to produce a substantially homogeneous moldable mixture. Hence, complex preforms consisting primarily of fibrous materials have not been made commercially to date by water-based molding methods because techniques to uniformly suspend fibrils in a water-based vehicle suitable for molding the preform have not previously been developed. Instead, most commercial fibrous preforms made using a water-based vehicle are made by vacuum forming, wherein fibrils are suspended in water and are drawn onto a porous mandrel by vacuum to build a large mat of fibrous material. The mat is subsequently dried and machined by conventional methods to produce complex-shaped preforms. This method is undesirable for high volume production because more fibril is deposited to make the mat than the final preform requires, machining is a costly way to shape the preform, and fibril density is undesirably not uniform throughout the preforms.